1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for, upon occurrence of a handover, minimizing data forwarding between Node Bs.
2. Description of the Related Art
The Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation asynchronous mobile communication system that uses Wideband Code Division Multiple Access (W-CDMA) and is based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), both of which are European mobile communication systems.
In the 3rd Generation Partnership Project (3GPP) in charge of the UMTS standardization, Long Term Evolution (LTE) is now under discussion as the next generation mobile communication system of the UMTS system. Service providers utilizing LTE, a technology for realizing high-speed packet-based communication having a data rate of a maximum of about 100 Mbps, aim at deployment of LTE approximately by year 2010. To this end, several schemes are under discussion, which include, for example, one scheme of reducing the number of nodes located in a communication path by simplifying configurations of the networks, and another scheme of maximally approximating wireless protocols to wireless channels.
FIG. 1 illustrates an exemplary configuration of the next generation mobile communication system. The system configuration shown herein is a configuration of the UMTS-based system.
Referring to FIG. 1, as illustrated, Evolved Radio Access Networks (E-RANs) 110 and 112 are simplified into a 2-node configuration of (Evolved Node Bs (or ENBs) 120, 122, 124, 126 and 128, and anchor nodes 130 and 132. A User Equipment (UE) 101 accesses the Internet Protocol (IP) network by means of the E-RANs 110 and 112.
The ENBs 120 to 128 each correspond to the existing Node B of the UMTS system, and are connected to the UE 101 by wireless channels. Unlike the existing Node B, the ENBs 120 to 128 perform complex functions. In LTE, because all user traffics including the real-time service such as Voice over IP (VoIP) are serviced over a shared channel, there is a need for an apparatus for collecting status information of UEs and performing scheduling depending on the collected information, and this operation is managed by the ENBs 120 to 128. One ENB generally controls a plurality of cells.
To realize a maximum data rate of about 100 Mbps, LTE is expected to use Orthogonal Frequency Division Multiplexing (OFDM) as a wireless access technology in a 20-MHz bandwidth. In addition, LTE will employ Adaptive Modulation & Coding (AMC) that adaptively determines a modulation scheme and a channel coding rate according to channel conditions of the UEs.
Many next generation mobile communication systems including LTE use Hybrid Automatic Retransmission reQuest (HARQ) as an error correction technique. HARQ is a technique of soft-combining previously received data with retransmitted data without discarding the previously received data, thereby increasing a reception success rate. More specifically, a receiving HARQ entity determines presence/absence of error in a received packet, and then sends an HARQ positive ACKnowledgement (HARQ ACK) signal or an HARQ negative ACKnowledgement (HARQ NACK) signal to a transmitting HARQ entity. The transmitting HARQ entity performs retransmission of the HARQ packet or transmission of a new HARQ packet according to the HARQ ACK/NACK signal. The receiving HARQ entity soft-combines the retransmitted packet with the previously received packet, thereby reducing a probability of error occurrences.
FIG. 2 illustrates a protocol stack of the LTE system.
Referring to FIG. 2, Packet Data Convergence Protocols (PDCPs) 205 and 240 each take charge of an IP header compression/decompression operation, and Radio Link Control (RLC) layers and 210 and 235 each reconfigure a PDCP Packet Data Unit (PDU) (hereinafter, a packet output from a particular protocol entity will be referred to as a ‘PDU of the protocol’) in an appropriate size, and performs an Automatic Retransmission reQuest (ARQ) operation thereon. As shown in FIG. 2, the PDCPs 205 and 240 are located in a UE and an Anchor node, respectively, and the RLC layers and 210 and 235 are located in the UE and an ENB, respectively.
Medium Access Control (MAC) layers 215 and 230, connected to several RLC entities configured in one UE, each multiplex RLC PDUs to a MAC PDU, and demultiplex RLC PDUs from the MAC PDU.
PHYsical (PHY) layers 220 and 225 each channel-code and modulate upper layer data into an OFDM symbol and transmit the OFDM symbol over a wireless channel. Further, the PHY layers 220 and 225 each demodulate and channel-decode an OFDM symbol received over a wireless channel, and forward the decoded data to the upper layer. Most HARQ operation of channel-decoding a received packet, soft-combining the channel-decoded packet with the previously received packet, and performing a Cyclic Redundancy Check (CRC) operation thereon is achieved in the physical layers, and the MAC layers control this operation.
FIG. 3 illustrates an example of an RLC operation.
As described above, the RLC layers and 210 and 235 guarantee reliable data transmission/reception through the ARQ process. With reference to FIG. 3, the ARQ process will be described in more detail. A transmission buffer 305 of a transmitting RLC layer (or an RLC layer in a transmitting entity) stores PDCP PDUs 310 until the transmission buffer 305 transmits the PDCP PDUs 310 to a receiving RLC layer (or an RLC layer in a receiving entity). The PDCP PDUs 310 are transmitted to the receiving RLC layer after they are reconfigured in an appropriate size in a framing block 315 and then a sequence number, increasing by increments of one, is added to each PDCP PDU 305, and the resulting RLC PDUs are buffered in a retransmission buffer 320 until an ACK signal is received from the receiving RLC layer.
The receiving RLC layer stores the received RLC PDUs in a reception buffer 330, recognizes a sequence number of a missing RLC PDU by checking the sequence numbers, and sends a request for retransmission of the missing RLC PDU to the transmitting RLC layer.
In the example of FIG. 3, RLC PDU[7]˜RLC PDU[10] are transmitted at a time, and among them, only the RLC PDU[7] and the RLC PDU[9] are received and stored in the reception buffer 330. The receiving RLC layer sends a status report 340, containing information indicating that it has normally received the RLC PDU[7] and the RLC PDU[9] and has failed to receive the RLC PDU[8] at an arbitrary time, to the transmitting RLC layer. Then, the transmitting RLC layer retransmits the retransmission-requested RLC PDU[8] stored in the retransmission buffer 320, and discards the normally transmitted RLC PDU[7] and RLC PDU[9].
FIG. 4 illustrates a data forwarding process during a handover.
When a UE makes a handover from a source cell of an ENB where it is currently located to a target cell of another ENB (hereinafter, an ‘inter-ENB handover’), the RLC entities are reconfigured in the target cell, so the source cell forwards the packets, whose transmission is not yet completed, to the target cell. The data 430 being forwarded from the RLC entity of the source ENB 420 to the RLC entity of the target ENB 425 during inter-ENB handover may include a non-transmitted PDCP PDU(s), a transmitted PDCP PDU(s), and an ACK signal which has not been received yet.
When an arbitrary PDCP PDU is transmitted in n RLC PDUs in a distributed manner, the PDCP PDU is not regarded as positively acknowledged, until ACK signals are received for all of the n RLC PDUs.
The RLC entity of the target ENB 425 transmits the non-transmitted PDCP PDUs and the un-ACKed transmitted PDCP PDUs to the UE, thereby preventing the packet loss that may occur during inter-ENB handover. However, because the transmission line used for inter-ENB data transmission is generally low in the data rate, it is preferable to minimize the inter-ENB data forwarding.